1. Field of the Invention
The invention relates to small lawn and garden vehicles having four wheel steering.
2. Description of the Related Art
It is known to provide small lawn and garden vehicles such as rider mowers with steerable front wheels and non-pivotable rear driven wheels. As the vehicle executes a turn, the front wheels pivot. The wheels tend to marr or scuff the ground if the axes of rotation of the four wheels do not intersect at a common point. Typically, lawn and garden vehicles such as rider mowers produce a relatively large amount of such steering errors. Also, the minimum turning radius of a vehicle having a two wheel steering mechanisms is relatively large.
Vehicles have been provided with four steerable wheels to thereby decrease the vehicle's minimum turning radius. According to a first type of four wheel steering mechanism, all four wheels are attached to the vehicle by respective pivotal couplings such that each wheel is capable of pivoting about a respective generally vertical axis. An arm is coupled with each pivotal coupling such that as the arm is pivoted the wheel pivots therewith to execute a turn. A tie rod extends between the front pair of arms for pivoting the two front wheels in unison. A similar tie rod extends between the rear pair of arms for pivoting the rear pair of wheels. A linkage is provided between the front and rear pairs of wheels such that all four wheels pivot at once.
When the vehicle is traveling straight forwardly, the tie rods and arms are positioned substantially perpendicular to each other. Therefore, the distance between the tie rod and the vertical axis defined by the pivotal coupling is relatively large, and a relatively large moment arm is produced for swinging the pivotal coupling during initiation of a turn. However, when the vehicle is executing a tight turn, the angle between the tie rods and the arms is relatively small, such that the distance between the tie rods and the vertical axis of the pivotal coupling is relatively small. Only a relatively small moment arm is available for pivoting the wheel during a sharp turn. Therefore, when the vehicle executes a relatively tight turn, a relatively large force must be applied to the tie rods in order to pivot the wheels from this position. Also, the wheels encounter ground forces that tend to urge the wheel to pivot back to an unpivoted or straight-ahead position. The tie rods keep the wheels in the desired pivotal positioned, and therefore the above mentioned ground forces are transmitted to the tie rods. Since the moment arms are relatively small during a tight turn, the forces encountered by the tie rods are relatively large. The tie rods and arms are thereby subjected to relatively large stresses and may fail or break.
Many of these types of four wheel steering mechanisms are designed such that steering errors are reduced only for one direction of turn.
A second type of four wheel steering mechanism provides a front plate pivotally coupled to the vehicle frame at the centerline of the vehicle. A pair of tie rods extend between the front plate and respective front wheels. A similar plate is pivotally coupled to the centerline of the frame at the rear of the vehicle. A linkage is provided that extends between the two plates to cause the plates to pivot simultaneously. The geometry of this type of mechanism allows relatively large lever arms to be maintained throughout even tight turns such that stresses in the tie rods and plates are relatively low. Therefore the likelihood of damaging a tie rod is correspondingly reduced. Furthermore, the symmetry that results from the use of the plates allows the mechanism to be designed such that steering errors are reduced an equal amount for turns in both directions.
Conventional mowing vehicles include a gear box or other means for transmitting rotational power from the vehicle engine to the mower deck. The gear box is often carried by the mower deck and generally beneath the vehicle. Other vehicle components such as the vehicle drive shaft, a PTO shaft for driving implements, and the attachment hardware and draft linkages that couple the mower deck to the vehicle are often positioned beneath the vehicle near the rear portion of the mower deck. Therefore, very little clearance is available for steering linkages to be positioned near the rear portion of the mower deck and beneath the vehicle. Furthermore, conventional mower decks are typically capable of being raised beneath the vehicle to transport positions such that the space between the deck and the vehicle is further reduced. The lack of vertical spacing between the deck and the vehicle is aggrevated by the presence of a laterally extending chute that is defined in the front portion of many conventional decks. A chute is often formed by the front of the deck to allow the grass that has been cut by the various mower blades within the deck to be channeled to one side of the deck for ejection through a discharge opening. The chute portion of the deck is taller and larger than the rear portion of the deck, and therefore, when the deck is raised as for transport the front chute portion of the deck may require a larger amount of space beneath the vehicle than the rear portion of the deck.
The plates and the linkages of the second type of four wheel steer mechanism discussed above tend to occupy a relatively large amount of space beneath the vehicle. In addition, the linkages and plates pivot and shift through a relatively large distance such that they require a relatively large amount of space during operation. These conventional mechanisms therefore may interfere with or prevent a mower deck from being operated beneath the vehicle between the front and rear wheels.
It would therefore be desirable to provide a vehicle with a four wheel steering mechanism that allows a mower deck to operate beneath the vehicle between the front and rear wheels. It would be desirable to provide a four wheel steering mechanism having front and rear pivotal members or plates coupled near the centerline of the vehicle with a pair of tie rods extending from each plate to respective pivotal wheels. It would be desirable for steering errors to be reduced, and stresses in the tie rods to remain relatively low throughout tight turning operations. It would be desirable to provide a linkage that operatively connects the plates, and that occupies only a relatively small amount of space during operation such that a mower deck can be operated beneath the vehicle between the front and rear wheels. In particular, it would be desirable to provide such a mechanism that accomodated the use of a mower deck having a chute formed in the front portion thereof. Furthermore, it would be desirable for such a linkage to be positionable beneath the frame to allow ease of access to the linkage for service. Such a linkage that allowed for the deck to be removed easily would also be desirable.